A single-mode, optical-fiber directional coupler couples a preselected fraction of the optical power from the single-mode guided radiation of the input optical fiber into the single-mode guided radiation of the output fibers to which it is coupled. A typical single-mode optical fiber consists of two concentric glass layers, the inner core layer being several microns in diameter and having one optical index of refraction and the outer concentric cladding layer having a second optical index of refraction. The cladding is in turn encased in a concentric protective, opaque jacket. The optical power flowing through the fiber is confined, by the diffracting-lens effect of the differences in indices of refraction, primarily within the center of the fiber to a radial distance no more than one or two core radii from the center of the fiber. Coupling is achieved by reducing the diameter of the cladding of each of two or more parallel fibers to bring the cores of the fibers closer to each other so that their modes overlap as desired.
Presently, the three most common types of single-mode, optical-fiber directional couplers are the lapped coupler, the biconical-tapered fused coupler, and the etched-cladding coupler. The lapped coupler is fabricated by imbedding an optical fiber in a channel in each of two separate quartz blocks. The blocks are lapped and polished until the cladding on the upper surface of each fiber is removed to the desired proximity to the cores. The two blocks are then pressed together to produce a parallel contact between the flat lapped surfaces of the two fibers. Sliding and rotating the two blocks tunes the degree of coupling. The lapping method can be used to produce good low-loss, relatively-thermally-insensitive, directional couplers but is costly to fabricate and results in a large, bulky coupler that is susceptible to misalignment in dynamic environments. The lapping method is limited to coupling between two fibers.
The biconical-tapered fused coupler is fabricated by first fusing two parallel fibers over a specified length at the softening temperature of the glass. The fibers are drawn under tension to reduce their diameters in the center of the fused region by 10-20% which results in a taper in diameter from the center toward the ends. The core of each fiber is itself reduced by 10-20% in diameter which weakens the diffracting-lens effect which results in a radially expanded region of confinement of the optical power such that a portion of the optical power is flowing in one or more of the guidance modes of the cladding. The cladding guidance modes in the input fiber overlaps the cladding guidance modes of the output fiber and a fraction of the optical power in the input fiber is thereby coupled to the output fiber. As the optical power propagates along the expanding tapers at the output ends of the coupler, it adiabatically redistributes from the cladding guidance modes back to the core guidance single mode at the output end of each fiber. The fused coupler has higher surface loss and loss from scattering centers within the cladding than lapped couplers and is highly sensitive to thermal and mechanical changes. Small imperfections in the claddings of both the input and output fibers upset the condition of adiabatic redistribution and lead to excessive radiation losses from these couplers. These types of couplers are very lossy compared to a typical lapped coupler. The biconical tapered coupler is not necessarily limited to two fibers.
Etched couplers are fabricated by removing the fiber jackets in the desired coupling region of several fibers and etching the claddings of the several optical fibers in the desired coupling region with hydrofluoric acid to reduce the diameters of the claddings of each of the fibers. The transition is usually abrupt from the etched region to the unetched region, usually located at the transition from jacketed to unjacketed fiber. The etched regions of the several fibers are brought into parallel contact and the contacting fibers are imbedded in a material that both matches the optical index of refraction of the cladding material and gives support and strength to the optical fibers.
There are both direct optical effects and mechanically induced optical effects associated with the abrupt transition from the diameter in the etched to the diameter in the unetched region. The etched coupling region of each fiber is usually less than 10.mu. in diameter so that a stress applied to the fiber causes a relatively sharp bend in the fiber at the transition point. Because lateral stress is required to bring the etched fibers into contact along the optical interaction segment, these undesirable sharp bends can be produced in the fibers at the transition points; as much as 40% of the incident optical power may be observed radiating from the abrupt transition region. Stress itself also changes the index of refraction in the transition area, causing additional optical mismatches between the guidance modes of the unetched and etched fiber regions, which produces further radiation loss.